Spin torque oscillators have been first demonstrated in the year 2003. See S. I. Kiselev et al., Nature 425, 380 (2003). Since then, they have been shown to provide a reliable option as microwave signal generators. They typically provide a smaller size than other microwave oscillators, and are defined mainly by the size of their ferromagnetic nanopillars, which typically have a size below about 200 nm. The frequency of a spin torque oscillator may be tuned by varying the current passed through the same. In general, the operation of spin torque oscillators relies on precession of magnetization in the free (or “active”) ferromagnetic (FM) layer under the action of spin torque due to electrons crossing the non-magnetic layer (such as copper or MgO, for example) from the FM layer. The variation or precession of magnetization is then converted to an electric signal via the effect of magnetoresistance, which refers to the change in resistance of the stack of materials of the spin torque oscillator based on the relative orientations of magnetization in the free and fixed FM layers.
Currently, microwave oscillators based on nanomagnets and their precession by the spin torque effect require an external magnetic field for their operation. Under such circumstances, one would need either a permanent magnet or a wire with current in order to create an external magnetic field. In the case of a permanent magnet, however, disadvantageously, the spin torque oscillator would take additional space, and further, interference would be created with other parts of the circuit. In the case of a wire, the arrangement would lead to constantly dissipated Joule heat, which could be disadvantageous to the circuit.